A) Field of the Invention
This invention relates to a method of introducing a required gas into a succession of containers being advanced along a path. Further, the invention relates to apparatus for introducing a required gas into such a succession of containers.
B) Description of the Invention Prior Art
In order to permit the internal treatment of a moulded plastics container, a pre-cursor to the treatment step might be the filling of the container with a particular gas required for the treatment step. At its simplest, the gas may be injected into the container through a suitable opening and under a sufficient pressure to ensure that enough gas enters the container for the container to be essentially filled with the gas.
A simple gas-filling process as described above is relatively inefficient and leads to significant loss of the required gas, during the filling step. In an attempt to minimise the loss of the required gas, a closure arrangement may be provided around a gas supply pipe and which connects with the opening to the container. The disadvantage of this is that such a closure arrangement will also restrict the displacement of air within the container prior to the filling of the container with the required gas. Consequently, it is difficult to obtain a sufficiently high proportion of the required gas in the container during a simple filling step as described above, unless the filling step is continued for a significant period of time, so also leading to much higher losses of the required gas.
The loss of the required gas becomes more significant if that gas is toxic, or if it is relatively expensive. For example, if a container is to be filled with carbon dioxide, the loss of carbon dioxide to atmosphere may not be that important. On the other hand, if a container is to be filled with a gas such as a halogen (e.g. chlorine), significant measures must be taken to prevent the loss of the chlorine to atmosphere, especially in a factory environment, in view of the toxic nature of the gas. With a noble gas (e.g. argon), the loss of the gas to atmosphere may be significant economic complications for the treatment process and so should be avoided, as far as possible.
It has been proposed to minimise the loss of the required gas whilst at the same time optimising the filling of a container with the gas by partially evacuating air from the container before introducing the required gas into the container. In this way, intermixing of the required gas with air may be reduced, and during the filling step, the amount of gas displaced from the container can essentially be eliminated. However, in the case of a moulded plastics container the evacuation of the container rarely is a possibility, since the container is likely to collapse leading to damage to the walls of the container.
The present invention has been developed to overcome the disadvantages of the usual processes adopted for filling a container with a required gas, especially for a industrial process where a succession of containers are to be filled, as a pre-cursor to a treatment step for the interiors of the containers.
Accordingly, one aspect of this invention provides a method of introducing a required gas into each of a succession of containers being advanced along a path, comprising feeding the required gas into a first container, collecting the gas displaced from the first container as the required gas is fed thereinto, and feeding the displaced gas into a following container on the path.
According to a second, but closely related, aspect of this invention, there is provided apparatus for introducing a required gas into each of a succession of containers being advanced along a path, comprising a conveyor to effect advancement of the containers through a plurality of gas-feeding stations, first supply means to feed the required gas into a container located at a first gas-feeding station, first collection means for gas displaced from the first container at the first station, and second supply means to feed the gas collected by the first collection means into a container located at a second gas-feeding station.
It will be appreciated that with the method or apparatus of this invention, a succession of containers being advanced along a path are filled with the required gas by at least a two-stage process, but preferably by a multi-stage process (and typically a six-stage process) where the concentration of the required gas in the containers is increased as the containers move along the path. Thus, in a typical method, the gas displaced from each of five containers (by the step of feeding gas into the respective container) is collected and, in each case, is fed into the next following container on the path. Then, the gas displaced from the last container to which displaced gas is fed (that is, the sixth container in said typical method) is allowed to discharge to atmosphere.
The performance of the method should be optimised such that the gas displaced from the last container of the succession is substantially all air. The gas displaced from the other containers will be mixtures of air and the required gas, with the concentration of the required gas rising between each successive pair of containers from the last container. The leading container of the succession, and to which the required gas is supplied, will displace an air/required gas mixture having the highest concentration of the required gas. By adjusting the operating parameters of the method, it is in this way possible to achieve a very high concentration of the required gas in the leading container, with the minimal loss of the required gas from the last container in the succession.